1. Field of the Invention
The present invention relates to an organic light emitting diode (OLED) display device, and more particularly, to an OLED display device having a high image quality.
2. Discussion of the Related Art
Generally, cathode ray tubes (CRT), one of display devices being widely used, are mainly used as monitors for televisions, measuring devices, information terminal devices, etc. However, due to CRT's weight and size, there is a limitation in minimizing the size and weight of these electronic products.
Recently, organic light emitting diode (OLED) display devices are being spotlighted as the next generation display device due to such advantages as high contrast ratio, high brightness, low consumption power, fast response time, wide viewing angle, lightweight, etc. The OLED display devices are also able to display various colors similar to natural colors and have a further advantage of a simple fabrication process. Thus, the OLED display devices are being widely used for mobile phones, personal digital assistants, computers, televisions, etc.
When a voltage driving method is adopted to drive the OLED display devices, the brightness of the OLED display devices is not uniform and it is difficult to display correct color images due to a sensitivity difference among R, G, and B colors. Therefore, a current driving method is mainly used to drive the OLED display devices.
FIG. 1 is a view illustrating an OLED display device according to the related art.
Referring to FIG. 1, the OLED display device includes a pixel array unit 115 having a plurality of pixels, a sample/hold unit 132 for supplying a current to the pixel array unit 115 to drive light emitting diodes provided in each pixel, and a digital/analogue (D/A) converter 134 for controlling an amount of the current supplied to the pixel array unit 115 from the sample/hold unit 132 in accordance with image data.
The pixel array unit 115 is provided with a plurality of transistors and light emitting devices (DD1). The light emitting device DD1 emits light by the current supplied from the sample/hold unit 132 via a transistor operated by a scan signal SS1. The light emitting device DD1 is generally a light emitting diode.
A power voltage source VDD is applied to the sample/holder unit 132, so that currents flow to the pixel array unit 115 and the D/A converter 134. An amount of the current introduced into the pixel array unit 115 and an amount of the current introduced into the D/A converter 134 have a 1:N relationship. More specifically, because the sample/holder unit 132 has a current mirror circuit, 1/N of the amount of the current flown to the D/A converter 134 from the sample/hold unit 132 is supplied to the pixel array unit 115.
The D/A converter 134 is provided for displaying images with various gray levels. The D/A converter 134 controls an amount of the current supplied from the sample/holder unit 132 in accordance with digital image data supplied from an outside video source. The D/A converter 134A includes a plurality of switching devices. As the switching devices are individually turned on or off in accordance with the digital image data, an amount of the current supplied to the D/A converter 134 from the sample/hold unit 132 varies. An amount of the current supplied to the pixel array unit 115 from the sample/hold unit 132 varies according to the amount of the current supplied to the D/A converter 134.
FIG. 2 is a view illustrating a circuit construction of the D/A converter and sample/hold unit of FIG. 1.
Referring to FIG. 2, a D/A converter 234 includes a plurality of switching devices SW1 to SW6 that are turned on or off according to digital image data D0 to D5, and a plurality of sink devices SNK1 to SNK6.
The switching devices SW1 to SW6 are turned on or off according to the digital image data D0 to D5. That is, the switching devices SW1 to SW6 are matched one to one to each bit of the image data D0 to D5. Because each bit of the image data D0 to D5 has a different weight value, an amount of the current that passes through each of the switching devices SW1 to SW6 is different. To do so, each switching device SW1 to SW6 has a different channel size. For example, because a weight value of the image data D0 corresponding to the first switching device SW1 is greater than a weight value of the image data D5 corresponding to the sixth switching device SW6, an amount of the current applied to the first switching device SW1 is greater than an amount of the current applied to the sixth switching device SW6.
The sink devices SNK1 to SNK 6 are operated by a bias voltage Vbias to control amounts of the currents applied to the switching devices SW1 to SW6. More specifically, when the sink devices SNK1 to SNK6 are turned on, they perform a current sink function for grounding the currents introduced to the switching devices SW1 to SW6, and when the sink devices SNK1 to SNK6 are turned off, they shield the currents applied from a sample/hold unit 232. Thin film transistors can be used as the sink devices SNK1 to SNK6 and the switching devices SW1 to SW6.
The sample/hold unit 232 includes two first transistors MP1 and MP2, two second transistors MN1 and MN2, and one capacitor C1. Because the second transistors MN1 and MN2 are N-type transistors, they are turned on when a control signal CS1 of a high voltage is applied thereto. Also, because the first transistors MP1 and MP2 are P-type transistors, they are turned on when the control signal CS1 of a low voltage is applied thereto. Accordingly, when the control signal CS1 is a high voltage, a current flows from the power voltage source VDD to the D/A converter 234 via the first transistor MP1 and the second transistor MN2, and the corresponding current also flows to a pixel array unit 215. Because the first transistors MP1 and MP2 provided in the sample/hold unit 232 have different channel sizes, an amount of the current applied to the first transistor MP1 and an amount of the current applied to the first transistor MP2 have a 1:N relationship.
Recently, a research is being actively conducted to make OLED display devices lighter and thinner. Accordingly, a driving circuit is integrally formed with a display panel using poly-crystalline silicon thin film transistors to reduce the fabrication cost. A low temperature poly-crystalline silicon (LTPS) method is mainly used to form the poly-crystalline silicon thin film transistors in which amorphous silicon is crystallized by laser.
In the LTPS method, when laser is non-uniformly irradiated on amorphous silicon or a surface treatment prior to irradiating laser is not properly performed, the characteristics of the poly-crystalline silicon thin film transistors become non-uniform, which negatively influences on the image quality or life span. That is, due to the irregular laser energy irradiated on amorphous silicon, the channels of the thin film transistors formed by the LTPS method have various sizes and distributions of crystallites. Accordingly, the carrier mobility and threshold voltage of each thin film transistor become different. Because the characteristics of the thin film transistors are different, an amount of current flown to each thin film transistor becomes different even when the same bias voltage is applied to an OLED display panel. Although the characteristics of the thin film transistors in different driving blocks may be largely different, the characteristics of the adjacent thin film transistors in one driving block are generally similar.
Accordingly, amounts of the currents applied to the sink devices SNK1 to SNK6 of each driving block are different even when the same bias voltage is applied to the OLED display device. Thus, even when the same image data is applied to each driving block, the same image is not implemented, thereby degrading the image quality of the OLED display device.